Drilling a well, and the subsequent completion operation, involves accessing a pressurised reservoir via a drilled bore. The primary means by which reservoir fluids from a well are prevented from escaping into the environment is by imposing a hydrostatic pressure on the well. This pressure is known as the primary barrier and is created by introducing an artificial fluid, mud or brine into the well during drilling or completion operations, through a marine riser which extends from the seabed to a surface vessel. The specific gravity or density of this fluid can be adjusted by careful control of the constituents making up the fluid mixture, which add or subtract weight to control the specific gravity. In this way the hydrostatic pressure can be adjusted to suit a particular well conditions, independent of the length of the fluid column between the surface and the reservoir.
Sometimes, the subterranean reservoir formation is weak and for this reason it is necessary to maintain the hydrostatic pressure at a minimum in order to prevent fracturing the formation. If fracturing occurs, fluids within the column can be lost into the formation which results in a reduction in the height of the fluid column and may lead to a situation in which the hydrostatic pressure becomes less than the well pressure. Particularly, in the case of drilling operations, if the well pressure exceeds the hydrostatic pressure there exists a possibility of losing control of the well. Initially this is felt as a “kick” in which well fluids begin to come to the surface slowly. If a kick is not detected immediately the well can quickly become out of control and is known in this state as a “blow out”.
To contain the well, a well containment system is provided between the marine riser and the wellhead. The well containment system comprises a secondary barrier system of invokeable seals on hydraulic rams installed onto the wellhead. The rams can be actuated to close in and seal the well. In order to do so whilst pipe is in the well, at least one of these rams is able to cut the drill pipe or other well tubulars, and seal the well to allow well control to be established. This part of the well containment system is known as the blow out preventer system. During completion operations the tool strings are passed down through the marine riser and well containment system into the well to allow downhole operations to be performed. In the event of a blow out, the rams have to be able to sever through the drill string, or, during completion operations, a high pressure internal riser and the tool string contained therein. In some cases this operation has either not been performed successfully because, for example, the high pressure riser is too tough for the blow out preventer rams to shear through.
Furthermore, when the tubular is sheared, the part of the tubular in the well drops downhole. Once it is safe to reconnect to the well, it is necessary to regain well control and to retrieve the dropped pipe. This combination of tasks is difficult and time consuming.
The marine riser is a large bore, low-pressure pipe which conducts the returning drilling fluid back to the mud storage system at surface. The well containment system also comprises an emergency disconnect package which is positioned between the bottom of the marine riser and the blow out preventer. The emergency disconnect package, which facilitates disconnection of the riser, isolates the marine riser from well fluids.
Two other conduits, known as the choke and kill lines, are strapped to the low-pressure marine riser. These are capable of withstanding high pressure and are used in the circulation of high-pressure fluids during the re-establishment of well pressure control.
The lines are connected to pipe pressure manifolds on the surface. The kill line is hooked up to the outlet of a high pressure pump and the choke line is hooked up to a high pressure choke which drops the pressure and allows fluids to return controllably to the mud storage system.
In deep water, the back pressure in the choke and kill lines becomes excessive to the extent it can become unacceptable. The increased length of these small bore lines creates a significant pressure increase due to friction and this can, in turn, apply a pressure to the well which may result in unnecessary formation breakdown. This problem has led to the development of a dual stack system, which includes a simplified blow out preventer stack with a disconnectable lower marine riser package attached to the wellhead. The lower marine riser package is connected to a large bore, high-pressure riser, the top end of which is connected to a near surface BOP stack, which in turn is attached to a short marine riser to surface.
A large bore, high-pressure riser is very costly and the cost increases dramatically with the increase in pressure and depth of water. As the size, complexity and weight of these systems increases the practicality for deployment of the systems from a surface facility, conventionally a ship, barge or rig, decreases.